(1) Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, printer, facsimile machine or the like and an image forming method using this apparatus.
(2) Description of the Related Prior Art
Conventionally, in image forming apparatuses based on electrophotography, the developers used for forming toner images on the photoreceptor drum are roughly classified into two types, namely, the mono-component type developer consisting of a single toner and the dual-component type developer containing a non-magnetic toner and a magnetic carrier.
Since the mono-component developer is suitable to make the system compact but is not suitable for high-speed development, dual-component developing systems are adopted in many cases for high-speed long-life image forming apparatuses. In the developing device using the dual-component developer, the carrier itself are not consumed from the dual-component developer but remains in the developing device without reduction. On the other hand, the toner is consumed and reduced by the operation of development. In this configuration, in order to prevent image quality from becoming unstable due to reduction of the toner that forms the dual-component developer, toner concentration control for supplying toner as appropriate to keep the toner concentration in the dual-component developer falling within a proper range is implemented.
Generally, toner concentration control is, in many cases, performed by using two kinds of control methods in combination. One is the method in which toner is supplied by calculating the amount of toner consumption based on the coverage ratio of input images. The other is the method in which the density of a standard toner image formed on the surface of the electrostatic latent image bearer (photoreceptor drum) is detected and toner supply is performed based on the comparison of the detection result with the predetermined density level.
As part for implementing the above toner concentration control, the image forming apparatus includes a means for detecting toner empty, e.g., a magnetic permeability sensor that detects the presence or absence of toner falling supplied into the developing device. Since the magnetic permeability sensor presents low sensitivity if the output voltage level falls out of the proper range, the detection accuracy of toner falling becomes lowered. For this reason, it is impossible to detect toner falling with precision unless the input gain of the magnetic permeability sensor is periodically adjusted to keep the output voltage level within the proper range.
To deal with this problem, there has been a technical proposal in Patent Document 1 (Japanese Patent Application Laid-open 2002-72661) in which toner concentration control is performed by calculating how the gains of the analog voltage detection values input to the magnetic permeability sensor are deviated from a predetermined value and correcting the output voltage level.
In the above toner concentration control, in the case of toner supply based on the coverage ratio, the calculated toner consumption from the count of pixels in the input image cannot represent the correct amount of the developer actually consumed, and it is also impossible to check whether the correct amount of toner has been supplied based on the calculated consumption of toner. Accordingly, if the amount of toner consumption and/or the amount of toner supply greatly deviate from the calculated value, depending on individuality of each image forming apparatus, this case poses a problem that the toner concentration falls out of the proper range so that the image quality of printed images degrades.
On the other hand, in the above toner control, when toner supply is performed based on the density of the standard toner image it is possible to some degree to compensate for the tendency of the toner concentration to deviate from the proper range, depending on individuality of each image forming apparatus. However, since the input gain of the magnetic permeability sensor is affected by change in toner concentration of the developer, change in fluidity and other physical properties of the developer and changes in temperature and humidity in the operating environment, even if correction is made based on the density of the standard toner image it is impossible to exactly correct the amount of toner consumption and the amount of toner supply, hence posing the problem that deviation of the toner concentration cannot be surely calculated.
Further, when the toner concentration falls out of the proper range, it is impossible to determine that the trouble is either a controllable error that is attributed to the individuality of each image forming apparatus or an uncontrollable error that is attributed to a fault of the image forming apparatus, hence posing the problem that it is impossible to suitably determine the operation of image printing to be permitted or to be prohibited.